Method of operating a transport refigeration unit

ABSTRACT

A method of detecting conditions during the operation of a transport refrigeration unit (20) which may cause shutdown, and then modifying the operation of the unit in an attempt to find an operating condition that will keep the unit operating safely. The unit includes a compressor (26) driven by an engine (30) operable at low and high speeds. Upon detecting (264) an over-temperature condition of the engine a first modification (282) is initiated, including switching the engine to low speed. If the engine temperature does not drop to a safe value (288, 292), a second modification (296) is initiated. If the engine was in low speed (280) when the over-temperature condition was detected, the second modification is initiated immediately. Upon detecting (314) a low oil pressure condition, the engine is switched (316) to high speed. If the oil pressure rises to a safe level, engine operation is continued at high speed. Appropriate alarms (326) accompany the modified operating conditions.

This is a division of application Ser. No. 07/728,465 filed Jul. 11,1991 now pending.

TECHNICAL FIELD

The invention relates in general to transport refrigeration units, andmore specifically to transport refrigeration units which havemicroprocessor based electrical control.

BACKGROUND ART

U.S. Pat. No. 4,663,725, which is assigned to the same assignee as thepresent application, discloses the use of microprocessor based transportrefrigeration control for use with a refrigerated container, with therefrigerant compressor being driven by an electric motor. This patent isdirected primarily to the use of a microprocessor to operate the variouscomponents of the refrigeration system according to predeterminedalgorithms, and to detect and record faults which occur during theoperation thereof.

U.S. Pat. No. 4,918,932, which is assigned to the same assignee as thepresent application, discloses the use of a microprocessor to determineaverage error between an operator selected set point temperature and thetemperature of a space to be conditioned, using the outputs of returnair and discharge air sensors. The average error is then used in thedetermination of an error signal which modulates the capacity of thesystem.

While these patents ably utilize the capabilities of a microprocessor incontrolling the operation of a transport refrigeration system, it wouldbe desirable, and it is an object of the present invention, to expandthe use of the microprocessor which controls the unit to provideadditional services in the area of unit fault conditions.

SUMMARY OF THE INVENTION

The invention is a method of modifying the operation of a refrigerationunit, when a condition is detected which may result in shutdown of theunit, in an attempt to find a safe operating condition which will keepthe unit operating. The refrigeration unit includes a refrigerantcompressor driven at low and high speeds by an internal combustionengine. The method includes the steps of monitoring the temperature ofthe engine, detecting a predetermined over-temperature condition, andinitiating a first modification phase which may result in modificationof the current operating mode of unit 20. The first modification phaseincludes the step of switching the engine to low speed, when it is inhigh speed at the time the predetermined over-temperature condition isdetected, and the step of determining if the engine temperature hasdropped below a predetermined value, after the step of switching to lowspeed. If the temperature does not drop below the predetermined value, asecond modification phase is initiated. If the engine is in low speedwhen the over-temperature condition is detected, the second modificationphase is initiated immediately.

In a preferred embodiment of the invention, the second modificationphase includes the step of switching the refrigeration unit to fullsuction line modulation, if the unit is not already in full modulation.

In another embodiment of the invention, the method includes the steps ofmonitoring the temperature and the level of the engine coolant, with thesteps of initiating the first and second modifications in the operationof the unit taking place only if the level of the engine coolant exceedsa predetermined value.

In another embodiment of the invention, the method includes the step ofmonitoring the engine oil pressure. Upon finding the engine oil pressurebelow a predetermined value, the method includes the steps ofdetermining if the engine is currently operating at the lower of the twooperating speeds. If the engine is operating at low speed, the methodattempts to prevent shutdown by the steps of switching the engine tohigh speed and determining if the oil pressure is still below thepredetermined value. If the engine oil pressure has now risen to, orabove, the predetermined value, instead of shutting the engine down, theengine is continuously operated at high speed. Appropriate alarms aregenerated to inform the operator of modified operating conditions.

In an embodiment of the oil pressure checking method, the invention alsochecks the oil level of the engine, with the step of switching theengine to high speed taking place only when the oil level is above apredetermined level. Finding the engine already at the higher of the twooperating speeds, or finding the oil level is low, results in the stepof initiating engine shut down.

Thus, in addition to detecting certain engine faults which may result inshutdown of the prime mover, and thus shutdown of the transportrefrigeration unit, the present invention sets forth methods whichmodify the operation of the engine and/or the associated refrigerationunit, in an attempt to find a modified operating condition of the engineand/or refrigeration unit which will prevent shutdown. If the engine isallowed to continue to operate in a modified operating condition,appropriate alarms are generated which notify the operator that one ormore operating conditions have been modified, and to check the reasonfor the modified operation. The alarms identify specific items whichshould be checked i.e., engine oil or engine coolant. If the engine isshut down, other alarms are generated which inform the operator as tothe nature of the cause which initiated shutdown.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more apparent by reading the followingdetailed description in conjunction with the drawings, which are shownby way of example only, wherein:

FIG. 1 is a partially block and partially schematic diagram of atransport refrigeration system having a refrigerant compressor driven byan internal combustion engine, which may utilize the methods of theinvention;

FIGS. 2A and 2B may be assembled to provide an electrical schematicdiagram of microprocessor based electrical control shown in block formin FIG. 1;

FIG. 3 is a flow diagram of a program which sets forth a microprocessorcontrolled temperature check of the internal combustion engine whichdrives the refrigerant compressor utilized in the transportrefrigeration system; and

FIG. 4 is a flow diagram of a program which sets forth a microprocessorcontrolled oil pressure check of the internal combustion engine.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawing, and to FIG. 1 in particular, there isshown a transport refrigeration unit 20 which may utilize the methods ofthe invention. Refrigeration unit 20 may be mounted on a container,truck, or trailer, such as on a wall 22 thereof, for example.Refrigeration unit 20 has a closed fluid refrigerant circuit 24 whichincludes a refrigerant compressor 26 driven by a prime mover arrangement28. Prime mover arrangement 28 includes an internal combustion engine30, and it may optionally include a stand-by electric motor 32. Engine30 and motor 32 are coupled to compressor 26 by a suitable clutch orcoupling 34 which disengages engine 30 while motor 32 is operative. Aselector 35 selects one of the two prime movers and provides an outputsignal to identify the selection.

Discharge ports of compressor 26 are connected to an inlet port of athree-way valve 36 via a discharge service valve 38 and a hot gas line40. The functions of three-way valve 36, which selects heating andcooling cycles, may be provided by two separate valves, if desired.Three-way valve 36 has a first output port 42, which is selected toinitiate a cooling cycle, with the first output port 42 being connectedto the inlet side of a condenser coil 44. Three-way valve 36 has asecond outlet port 46, which is selected to initiate a heating cycle, aswill be hereinafter described.

When three-way valve 36 selects the cooling cycle output port 42, itconnects compressor 26 in a first refrigerant circuit 48, which inaddition to condenser 44, includes a one-way condenser check valve CV1,a receiver 50, a liquid line 52, a refrigerant drier 54, a heatexchanger 56, an expansion valve 58, a refrigerant distributor 60, anevaporator coil 62, an optional controllable suction line modulationvalve 64, another path through heat exchanger 56, an accumulator 66, asuction line 68, and back to a suction port of compressor 26 via asuction line service valve 70. The operative prime mover may beprotected against overload by controlling modulation valve 64 to providethe function of a conventional compressor throttling valve, as taught byU.S. Pat. No. 4,977,751, which is assigned to the same assignee as thepresent application; or, a conventional compressor throttling valve maybe disposed in suction line 68, as desired. Expansion valve 58 iscontrolled by a thermal bulb 71 and an equalizer line 73.

When three-way valve 36 selects the heating cycle output port 46, itconnects compressor 26 in a second refrigerant circuit 72. The secondrefrigerant circuit 72 by-passes condenser 44 and expansion valve 58,connecting the hot gas output of compressor 26 to the refrigerantdistributor 60 via a hot gas line 74 and a defrost pan heater 76. A hotgas by-pass solenoid valve 77 may optionally be disposed in hot gas line74. A by-pass or pressurizing line 78 connects hot gas line 74 toreceiver 50 via by-pass and check valves 80, to force refrigerant fromreceiver 50 into an active refrigerant circuit during heating anddefrost cycles.

A conduit or line 82 connects three-way valve 36 to the low side ofcompressor 26 via a normally closed pilot solenoid valve PS. Whensolenoid valve PS is deenergized and thus closed, three-way valve 18 isspring biased to select the cooling cycle output port 42. Whenevaporator 62 requires defrosting, and when the load being conditionedrequires heat to maintain set point, pilot solenoid valve PS isenergized to allow the low pressure side of compressor 26 to operatethree-way valve 36 to select the heating cycle output port 46.

A condenser fan or blower (not shown) causes ambient air 84 to flowthrough condenser coil 44, with the resulting heated air 86 beingdischarged to the atmosphere. An evaporator fan or blower (not shown)draws air 88, called "return air", from a served space 90 whose air isto be conditioned, through the evaporator coil 62, and the resultingcooled or heated air 92, called "discharge air", is returned to thespace 90. During an evaporator defrost cycle, the evaporator fan orblower is not operated, and a defrost air damper may be operated toclose the discharge air path to the conditioned space 90.

Transport refrigeration unit 20 is controlled by microprocessor basedelectrical control 94 which includes a microprocessor 96 and electricalcontrol 98. Electrical control 98 includes relays, and the like, as willbe explained relative to FIGS. 2A and 2B. The microprocessor 96 receivesinput signals from appropriate sensors, such as from a return airtemperature sensor 100 disposed in a suitable return air path 102, adischarge air temperature sensor 104 disposed in a suitable dischargeair path 106, from a coil temperature sensor 108 disposed to sense thetemperature of the evaporator coil 62, from a refrigerant pressuresensor (HPCO) 110 disposed on the high side of the refrigerant circuit48, and from various engine sensors shown in FIG. 2B, such as oil levelsensor 112 oil pressure sensor 114, engine coolant level sensor 115,engine coolant temperature sensor 116, and engine speed sensor 118.

Microprocessor 96, among other things, controls modulation valve 64, hotgas solenoid valve 77, and a throttle or high speed solenoid 120. Otherfunctions controlled by microprocessor 96 are shown in FIGS. 2A and 2B,and will be hereinafter described.

FIGS. 2A and 2B may be assembled to provide a detailed schematic diagramof microprocessor based electrical control 94, which includesmicroprocessor 96 and control 98. As is well known, microprocessor 96includes a read-only memory (ROM) 122 for storing programs to behereinafter described, and a random access memory (RAM) 124 for softwaretimers, flags, input signals, output signals, and other values generatedby the operating programs. Microprocessor 96 also includes a display 125for displaying fault codes, system status indicating lights, and thelike.

Electrical control 98 includes a battery 126 which has one sideconnected to a first conductor 128 via a DC shunt 130, an on-off switch132, and normally closed contacts 134 of a protective reset switch SSW.The remaining side of battery 126 is connected to conductor 136, whichis grounded. Control 98 further includes an alternator 138 driven byprime mover 28; a starter motor 140, for cranking engine 30, which iscontrolled by a starter solenoid 142 having associated normally opencontacts 143, an ignition switch 144, and a start relay 146 havingassociated normally open contacts 147; and glow plug resistors (GP) 148,for pre-heating engine 30, which are controlled by a pre-heat switch 150and by a pre-heat relay 152 which has normally open contacts 153.

Control 98 also includes a three-position switch 154 which has two banksof three terminals each comprising a center terminal and upper and lowerterminals, with reference to FIG. 2A. Switch 154, in the illustratedupper position which connects the center terminal to the upper terminal,places unit 20 under control of the microprocessor 96. The upperposition provides voltage from conductor 128 to a conductor 155. Anintermediate position of switch 154, in which the center terminal is notconnected to either the upper terminal or the lower terminal, isselected when the microprocessor 96 is not utilized and the load in theconditioned space 90 is frozen. This switch position will cause unit 20to operate continuously in a low speed cool mode. The lower position ofswitch 154 is selected when the microprocessor 96 is not utilized andthe load in the conditioned space is fresh. This position of switch 154will cause unit 10 to operate continuously, cycling between heating andcooling cycles under the control of the hereinbefore mentioned coiltemperature switch 108. Coil temperature switch 108 is preset to closeat a predetermined coil temperature, such as 35° F., to energize thepilot solenoid PS and initiate a heating cycle, and to open at apredetermined higher temperature, such as 38° F., to de-energize pilotsolenoid PS and initiate a cooling cycle.

In addition to the relays already mentioned, control 98 includes ashutdown relay 156, a run relay 158, a heat relay 160, a high speedrelay 162, a defrost damper relay 164, and a hot gas relay 166. Shutdownrelay 156 is normally energized, and is de-energized to shut unit 10down via its associated set of normally-closed contacts 168 which groundthe protective switch SSW and cause it to open its contacts 134. The runrelay 158 has normally-closed and normally open contacts 170 and 172,respectively, connected to a mode selector switch 174 which has an inputconnected to conductor 128. Selector switch 174 selects either acontinuous operating mode in which the prime mover 28 operatescontinuously, or a cycling startstop mode, also called "cycle sentry",which includes starting and stopping the prime mover 28.

The normally-closed contacts 170 of run relay 158 are connected to the"continuous" position of selector switch 174, and the normally-opencontacts 172 of run relay 158 are connected to the "cycling" position ofselector switch 174. Contacts 170 or contacts 172 provide voltage to aconductor 175 from conductor 128 and selector switch 174.

Heat relay 160 has a set of normally open contacts 176 for controllingthe pilot solenoid PS. High speed relay 162 has a set of normally opencontacts 178 for controlling the high speed solenoid 120. Damper relayhas a set of normally closed contacts 180 and a set of normally opencontacts 182, connected to control a defrost damper solenoid 184. Hotgas relay 166 is provided for controlling the hot gas solenoid valve 77via a set of normally open contacts 186, when a hot gas solenoid 77 isprovided in hot gas line 74.

Control 98 also includes a engine coolant temperature switch (high watertemperature -HWT) 190, which closes when the engine coolant reaches apredetermined elevated temperature, and a low oil pressure switch (LOPS)192 which is open as long as engine pressure is normal. The closing ofeither switch 190 or 192 will shut unit 20 down via the manual resetswitch SSW.

Microprocessor 96 senses the voltage across DC shunt 130 via conductors194 and 196, and can thus determine the magnitude and polarity ofbattery current. One polarity, which will be called positive, indicatesthe battery 126 is being charged by alternator 138, which also indicatesthe prime mover 28 is running. The other polarity, i.e., negative,indicates the battery is discharging.

Microprocessor 96 also has a conductor 198 which senses the position ofthe low oil pressure switch 192, conductors 200 and 202 which sense thevoltage level on first and second sides, respectively, of the highrefrigerant cut-out switch 110, a conductor 204 which senses whether ornot a modulation valve selector jumper 206 has connected conductor 204to system ground 136, a conductor 208 which senses whether or not adefrost sensor switch 210 has operated, signifying the need for adefrost cycle, and a conductor 211 which detects voltage on the dampersolenoid 184.

Microprocessor 96 has a plurality of output conductors for controllingvarious functions, including conductors 212, 214, 216, 218, 220, 222,224 and 226 for respectively controlling the operation of start relay146, pre-heat relay 152, shutdown relay 156, damper relay 164, highspeed relay 162, run relay 158, heat relay 160, and hot gas relay 166. Aconductor 228 is also provided for controlling the current level in themodulation valve 64.

Referring now to FIG. 3, there is shown an engine temperature monitoringprogram 230 which sets forth certain teachings of the invention. Program230 is entered at 232 and step 234 determines if engine 30 has beenselected as the prime mover, and if so, step 234 also determines ifengine 30 running. If electric motor 32 has been selected as the primemover, or engine 30 has been selected but it is not running, step 234exits program 230 at 236.

If step 234 finds engine 30 is the prime mover, and engine 30 isrunning, step 238 determines if the temperature of engine 30 exceeds apredetermined maximum value. In the exemplary embodiment of theinvention, engine 30 is liquid cooled and the temperature of engine 30is monitored by water temperature sensor 116 shown in FIG. 2B. Otherengine parameters which could be monitored to determine enginetemperature include the temperature of the engine block and thetemperature of the engine exhaust gases.

The predetermined maximum temperature value used in step 238 is selectedsuch that if the water temperature exceeds this value, e.g., 220° F., itmay not be necessary to run portions of program 230 related to detectingconditions which may lead to shutdown, as with an engine temperatureabove the selected maximum value, shutdown may be imminent.

If step 238 finds the water temperature above 220° F., step 240 sets ashutdown flag SDF true, which will result in the shutdown of engine 30,and thus unit 20, if program 230 is exited with flag SDF true. Step 240also sets an alarm HWT, indicating with an appropriate code in display125 the exact cause of shutdown, i.e., high engine water temperature.

Before exiting program 230 with SDF true, however, program 230determines if the high engine water temperature is a transitorycondition which will shortly drop to a safer level. To determine this,time is provided for the engine temperature to drop below apredetermined value, e.g., below 218° F. To implement this feature, step240 also starts a timer, such as a software timer in RAM 124. Step 242updates the timer and the program does not advance until either theengine temperature drops below 218° F., or a predetermined period oftime has elapsed, e.g., 25 seconds.

More specifically, step 244 determines if the engine water temperaturehas dropped below 218° F., and step 246 detects when the timer startedin step 240 reaches 25 seconds. Steps 218 and 246 loop back to the timerupdating step 242 until either step 244 or step 246 find "yes" answersto the questions posed by the steps, and step 248 then determineswhether the program arrived at this point via step 244 or step 246.

If the timer reached 25 seconds, the temperature did not drop below 218°during the 25 second time period, and the true shutdown flag SDF remainstrue as the program exits at 236 after step 250 turns on a shutdownindicating light on display 125.

If step 248 finds the timer did not reach or exceed 25 seconds, thenstep 244 found that the water temperature dropped below 218° F. Step 248is then entered which clears the alarm HWT which was set in Step 240.Step 254 determines if there are any other active alarms which will, orhave, shut unit 20 down. If there are no other shutdown alarms, step 256sets the shutdown flag SDF false, and step 256 also energizes shutdownrelay 156, in the event unit 20 had already been shut down. If step 254finds some other shutdown alarms exist, they are allowed to remain, withboth step 256 and the "yes" branch from step 254 going to step 258.

Step 258 determines if the engine water temperature is below apredetermined value, e.g., 190° F., with this value being a dividingline which separates normal engine temperature from a water temperaturerange which borders a temperature value which indicates a potentialwater temperature problem. If step 258 finds the water temperature isbelow 190° F., step 260 determines if there are any water temperaturerelated alarms which have not been cleared. If there are, with theengine water temperature now below 190° F. these set alarms may becleared. Accordingly, step 262 clears alarm UFTLS "unit forced to lowspeed", and it also clears alarm UFTFM "unit forced to full modulation".Step 262 then turns off the alarm light in display 125, and the programexits at 236.

If step 258 finds that the water temperature exceeds 190°, step 264determines if the water temperature is high enough, e.g., 210° F., thatmodification of the operating mode of unit 20 should be considered, toprevent an eventual shutdown of unit 20 because of excessive enginetemperature. If step 264 finds that the water temperature is between190° and 210° F., the water temperature is still in a zone which doesnot require modification of the operating mode of unit 20, and theprogram exits at 236.

If the engine temperature continues to rise and a subsequent running ofprogram 230 finds that the water temperature is equal to, or higher than210°, then program 230 enters a phase to determine if the elevatedengine water temperature is transitory. Step 266 starts a software timerin RAM 124, step 268 updates the timer, step 270 checks to see if thewater temperature has dropped to, or below, a predetermined value below210°, such as 205° F., and step 272 limits the amount of time which hasbeen provided to determine if the engine water temperature is going todrop. Except for the values of the water temperature and time, steps266, 268, 270 and 272 are similar to the hereinbefore described steps240, 242, 244 and 246.

If the water temperature drops below 205, detected by step 270, or thetimer reaches or exceeds 30 seconds, detected by step 272, the programarrives at step 274 which determines whether step 270 or step 272 causedthe program to reach step 274. If the timer did not reach thepredetermined value, e.g., 30 seconds, then the water temperaturedropped below 205° F. and the program exits at 236. If the timer reachedor exceeded 30 seconds, then the water temperature did not drop below205° F. and step 274 goes to step 276 which sets an alarm "check enginewater temperature" (CEWT).

In a preferred embodiment of the invention, before proceeding with theprogram portion devoted to finding modified operating modes which maykeep unit 20 running, the engine water level is checked in step 278 viawater level sensor 115, to determine if the level is below apredetermined safe level. Sensor 115 may simply be of the type whichprovides a signal when the water level is below a predetermined level.If the water level is low, as determined by step 278, then alternativeoperating modes will not be of benefit, and the program exits at 236.The hereinbefore described step 238 will eventually start the shutdownof engine 30 and unit 20, if the low water level problem is notcorrected. Alarm CEWT, generated in step 276, will alert the operator tocheck the engine water level.

If step 278 does not find that the engine water level is low, then step278 advances to step 280 which checks high speed relay 162 to determineif engine 30 is currently running at high speed, e.g., 2200 RPM, or lowspeed, e.g., 1400 RPM. If engine 30 is running at high speed, program230 starts a first modification phase with step 282. Step 282de-energizes high speed relay 162, and its normally open contacts 178open to de-energize high speed solenoid 120 and drop engine 30 to thelow speed setting. Step 284 then continues the first modification phaseby setting the hereinbefore mentioned alarm UFTLS, which notifies theoperator that the operation of unit 20 has been modified by forcing itto low speed. Step 284 also turns on the alarm indicator light indisplay 125, so the operator's attention will be directed to the alarmcode portion of display 125.

Program 230 then provides time for the first phase modification to work,by setting a timer in step 284. The timer is updated in step 286. Step288 detects the dropping of the engine water temperature into the saferange, i.e., below 190° F. Step 290 terminates the test period if theengine water temperature does not fall into the safe range after apredetermined period of time, e.g., 5 minutes. Step 292 determineswhether step 288 or step 290 caused the program to break out of the loopwhich includes steps 286, 288 and 290. If step 292 finds that the watertemperature dropped into the safe range, then the program exits at 236.The hereinbefore described step 262 will clear alarm UFTLS during thenext running of program 230.

If step 292 finds that the timer reached 5 minutes, then the enginewater temperature did not drop into the safe range, and program 230starts a second modification phase to further modify the operation ofunit 20 in an attempt to provide an operational mode which will preventan eventual shutdown of unit 20 due to high engine water temperature.This second phase starts with step 294 which determines if the presentoperational mode is "low speed, full modulation". Full modulationindicates that suction line modulation valve 64 is fully closed, withfull modulation reducing the load on engine 30. If the program arrivedat step 294 from step 282, the operational mode should be "low speed",and the microprocessor checks to see if it is providing a signal whichcauses the modulation valve 64 to fully close. If suction linemodulation valve 64 is fully closed, then unit 20 is already in lowspeed, full modulation, and the program exits at 236. If the enginewater temperature continues to rise while unit 20 is in low speed, fullmodulation, then the hereinbefore described step 238 will eventuallystart shutdown of unit 20.

If step 294 finds that unit 20 is not in a low speed, full modulationoperating mode, the second phase of the operating condition modificationquest continues, with step 296 causing microprocessor to provide acurrent sinking path for current through modulation valve 64 whichincreases the modulation valve current to the magnitude required tofully close valve 64. Step 296 also disables the high speed relay 162,to prevent some other program from switching engine 30 to high speed.Step 296 also sets the hereinbefore mentioned alarms UFTLS and UFTFM, itturns the alarm indicator light in display 125 on, and the program exitsat 236. As hereinbefore stated, if the operational mode modificationsinitiated by program 230 do not keep the water temperature fromeventually reaching 220° F., the hereinbefore described step 238 willinitiate the shutdown process.

If step 280 finds that engine 30 is already in low speed then the firstmodification phase is skipped, and the second modification phase isimmediately entered, with step 280 going to the hereinbefore describedstep 294.

The methods of the invention also include monitoring engine oilpressure, with FIG. 4 setting forth a program 300 for monitoring engineoil pressure. Program 300 is entered at 302 and step 304 determines ifengine 30 is the selected prime mover, and that it is running. If not,the program exits at 306.

If engine 30 is selected as the compressor prime mover, and engine 30 isrunning, step 308 checks to see if the engine oil pressure is below apredetermined value, e.g., 15 psi, which gives rise to concern. Oilpressure sensor 114 may be of the type which provides an input tomicroprocessor which indicates whether or not the engine oil pressure isbelow the predetermined value. If the engine oil pressure is not below15 psi, the oil pressure is O.K. and program 300 exits at 306.

If step 308 finds that the engine oil pressure is below 15 psi, thenstep 310 determines if modification of the current operating mode ofunit 20 may be beneficial. Step 310 checks the high speed relay 162 todetermine if engine 30 is running at the low or high speed settings ofthe engine throttle. If engine 30 is running at high speed, thenmodification of the operating mode of unit 20 would not be beneficial,and step 312 sets alarm LOP, indicating low engine oil pressure.Shutdown flag SDF is also set true in step 312, as continued operationwith low oil pressure may damage engine 30. Step 312 also turns on analarm indicator light in display 125.

If step 310 finds that engine 30 is operating in low speed, then, if theoil level is not low, the engine oil pressure may possibly be raised bygoing to the high engine speed setting. Step 314 first checks the oillevel sensor 112 in step 314 to detect a low oil level. Oil level sensor112 may be of the type which provides a predetermined output when theengine oil level drops below a predetermined level. If oil level sensor112 indicates that the oil level is low, then step 314 goes to step 312to start the shutdown process.

If the oil level is not low, step 314 continues the modification phaseof program 300 by going to step 316 which energizes high speed relay162. Normally open contacts 178 of high speed relay 162 close, and highspeed solenoid 120 is energized to move the engine throttle to the highspeed setting. Program 300 then provides time for the engine oilpressure to increase after the switch from low to high engine speed, bystarting a timer in step 318, updating the timer in step 320, anddetermining when a predetermined time period has elapsed, e.g., 5seconds. After 5 seconds, step 324 checks oil pressure sensor 114 todetermine if high speed engine operation has raised the engine oilpressure to, or above, the oil pressure setting, which is 15 psi in theexample. If the modification of the operating mode from low speed tohigh speed did not raise the oil pressure out of the danger zone, thenstep 324 goes to the hereinbefore described step 312 to start theshutdown process.

If step 324 finds the engine oil pressure has been raised to, or above,15 psi, then program 300 allows engine 30 to operate at high speed,exiting at 306 after setting alarms CEOP and UFTHS, and turning the onthe alarm light in display 125. Alarm CEOP alerts the operator to checkthe engine oil pressure, and alarm UFTHS notifies the operator that theunit has been forced to high speed operation.

I claim:
 1. A method of modifying the operation of a refrigeration unit when a condition is detected which may result in shutdown of the unit, with the refrigeration unit having a refrigerant compressor driven at low and high speeds by an internal combustion engine, with the engine including an oil pressure sensor, comprising the steps of:monitoring the oil pressure of the engine, detecting a predetermined low oil pressure condition, switching the engine to high speed when it is in low speed at the time the detecting step detects the predetermined low oil pressure condition, determining if the engine oil pressure has increased above a determined value after the step of switching to high speed, continuing to operate the engine at high speed when the determining step finds the engine oil pressure has increased above the predetermined value, and shutting the engine down when the determining step finds the engine oil pressure did not increase above the predetermined value.
 2. The method of claim 1 including the step of providing time after the step of switching the engine to high speed, before initiating the step of determining if the engine oil pressure increased above the predetermined value.
 3. The method of claim 1 wherein the engine has an oil level sensor, and including the steps of monitoring the level of the engine oil, and detecting when the oil level is below a predetermined level, with the step of switching the engine to high speed taking place only when the oil level of the engine is at or above the predetermined level. 